The purpose of this project is to study the physical, chemical, and ultrastructural properties of calcium phosphate salts, and to clarify the kinetic and thermodynamic processes and the interactions with substances of biological interest that uniquely enable calcium phosphate salts to carry out their specialized role in vivo. The properties of calcium phosphate salts are being studied with a variety of ultrastructural and physical-chemical techniques such as spectroscopic methods, x-ray diffraction, surface area analyses, chromatographic and standard analytical chemistry procedures. Currently, the principal effort under this project is to study the physicochemical effects biologically important ions such as fluoride, carbonate, organic anions such as citrate, and magnesium have on the texture, i.e., size/shape, of apatite crystals grown in vitro under constant physiological-like solution conditions. The aim of this study is to distinguish the direct effect these solution substances have on the texture of biological apatites from that brought about by the metabolic and matrix changes these substances induce in vivo. The first results from this study showed that solution supersaturation was a critical factor in establishing whether crystal growth or crystal proliferation is the dominant crystallization mechanism in apatite precipitation reactions in vitro. As supersaturation increased, growth was supplanted by proliferation. More recent findings showed that specific solution ions also influenced the mode of apatite formation in vitro, apart from their effect on supersaturation. Carbonate was found to be the primary reason why apatite formation in physiological-like aqueous solutions occurs by a proliferative mechanism. For this reason, carbonate could be a significant factor in controlling the size of apatite crystals in skeletal tissues. Fluoride and magnesium ions were found to partially offset the repressive effect carbonate had on primary crystal growth. However, these two ions, especially fluoride, stimulated crystal growth in an anisotropic manner. Most noticeable was increased growth in width/thickness with no measurable change in length. This anisotropy was similar to that observed in fluoridated adult human bone apatite crystals and suggests that fluoride ions at physiologically relevant concentrations may affect crystal growth in vivo directly by selectively altering the growth kinetics of the crystal surfaces rather than indirectly through the cellular and metabolic effects they may have on bone tissue development.